cycloalkanes : have one or more rings of carbon atoms cycloalkanes are alkanes that have carbon...

Cycloalkanes : have one or more rings of carbon atoms

•Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)

• Nomenclature

cycloalkanes

3

Naming CycloalkaneCount the number of carbon atoms in the ring and the number in the

largest substituent chain. If the number of carbon atoms in the ring is

equal to or greater than the number in the substituent, the compound

is named as an alkyl-substituted cycloalkane

For an alkyl- or halo-substituted cycloalkane, start at a point of

attachment as C1 and number the substituents on the ring so that the

second substituent has as low a number as possible.

Number the substituents and write the name

األحادية( 1• المفتوحة الروابط المركبات حرة في تكونمحورها حول الدوران

في( 2• األحادية الروابط وكذلك والثالثية الثنائية الروابطمحورها حول الدوران حرة تكون ال الحلقية المركبات

لأللكانات( 3• هندسية متماكبات توجد فال ولذلكهندسية متماكبات توجد بينما السلسلة المفتوحة

الحلقية لأللكانات

Conformation of Alkanes

• Straight chain C-C (free rotation) -Cyclic (no free rotation) • There are two different 1,2-dimethylcyclopropane isomers, one

with the two methyls on the same side (cis) of the ring and one with the methyls on opposite side (trans)

Geometric IsomerismCis-Trans Isomerism in Cycloalkanes

Reaction of cycloalkanes• Ring less stable

• Ring more stable 5 and 6

HICH3CH2CH2I

H2SO4 / H2OCH3CH2CH2OH

H2/Ni

Heat or UVCH3CH2CH2CH3

Br2/CCl4AlBr3

BrCH2CH2CH2CH2Br

CH3

Br2/UV or Heat

CH3

Br

Cl2/heat or UVCl

• Conformations: The temporary molecular shapes that result from rotations of groups about single bonds.

• Conformational analysis: the analysis of the energy changes that a molecule undergoes as groups rotate about single bonds

Conformations and conformational analysis of

Alkanes

األشكال • هي السلسلة المفتوحة لأللكانات الفراغية الهيئةنتيجة الفراغ في الجزيء ذرات تتخذها ان يمكن التي المختلفة

األحادية الرابطة حول للدوران•: الطرق تلك باحدى الفراغ في الجزيئات وتمثل

•1: نيومان( اسقاط

األمامية الذرات الخلفية الذرات

ساوهورس( 2• اسقاط

وتمثل األجناب، احد من الجزيء إلى ينظر هناالمقطعة بالخطوط الخلف من الروابط

أما السميكة بالخطوط األمام من والروابطبالخطوط فتمثل النظر مستوى في الروابط

المستمرة

Conformation analysis in Ethane

Eclipsed conformation0o

Staggered conformation for Ethane

60o

Conformation analysis in Propane

Conformation analysis in Butane

تمارينإسقاط • من منيو أرسمي لكل :-ان

لـ – • ثبات األكثر الصورة برومو – 1أإيثان

ثباتا – • واألكثر ثباتا األقل الصورةالكربون إلى النظر عند رقم ةللبيوتان

2.

• Cyclohexane Ring Stereochemistry

1 2Chair conformation Boat conformation

The most stable conformation of the cyclohexane ring is the “chair” conformation

1) The C—C bond angles are all 109.5°

. هيئة ولتحول القارب وضع من طاقة أقل الكرسي وضعحوالي طاقة وضع من البد القارب هيئة الى 6الكرسي

Kcal/mol

• Part I. How do you draw the cyclohexane ring and position the

substituents?• Start by drawing the ring as one of the following:

Chair conformation of cyclohexane

محورية رابطة لها كربون ذرة بالرمز ((axial(كل لها يرمز على( aو عموديةمحيطية وأخرى بالرمز (( equatorial(المستوى لها يرمز للمستوى( eو موازية

ea

محيطية (e)( equatorial)رابطةمحورية (a )(axial)رابطة

• Now you need to place the axial substituent bonds. These bonds will point upward towards the top of the page or downwards towards the bottom of the page (3 in each direction). The direction of the bonds alternate up-down-updown-up-down as you go from one carbon to the next:

• Now that the axial bonds are in place, the equatorial become much easier if you keep one thought in mind - “bondangles around C with 4 bonds are optimally ~109.5 degrees”. Draw the equatorial bonds so that they form a 109.5 deg angle (i.e. >90 deg) with the axial bond drawn above. Make sure they point a little up or a little down rather than pointing directly towards the left or right margin. This will be extremely helpful latter when we determine the cis-trans relationship between substituents.

When the ring flips, all of the bonds that were axial become equatorial

Part II. Determining the position of a substituent in the most stable conformer.

• The axial positions above the ring point into the same space as is true for the axial positions below the ring (see figure) thereby putting the substituents very close together. However, the equatorial bonds point out away from the ring providing much more room for the substituents (compare below).

Therefore, if possible all substituents would prefer to be in equatorial positions. However, if one is in the equatorial and one is

axial, preference would be given to the larger group being equatorial.

Part III. Cis-trans relationship between cyclohexane substituents

• cis - refers to two substituents being on the same side of the ring (or double bond)

• trans - refers to two substituents being on the opposite side of the ring (or double bond)

Relationship between Substituents Carbons

Axial - Axial Axial - Equatorial Equatorial - Equatorial

1 and 2 Trans Cis Trans

1 and 3 Cis Trans Cis


1 and 5 Cis Trans Cis
